1. Field of the Invention
The present invention relates to a method of manufacturing diamond devices, and more particularly to a method of manufacturing thermistors having high temperature coefficients utilizing diamond films deposited by vapor phase reaction.
2. Description of the Prior Art
There have been two types of thermistors, i.e. PTC (positive temperature coefficient) devices and NTC (negative temperature coefficient) devices. The former are made of barium titanate and the later of silicon carbide for example. The temperature range in which these conventional devices can operate is not so wide and their response speed to temperature change is not so high.
On the other hand, electric devices utilizing diamond have recently attracted researcher's interest. Some attempts have been made to form thermistors by the use of diamond film as a thermally sensitive area. The prior art diamond thermistors have only small thermistor coefficiently and require relatively high voltages to be applied thereacross. The inventor carefully investigated the thermal characteristics of the prior art thermistors. The thermistor coefficients thereof were measured to be as large as about 7000 (activation energy=0.6 V) when the diamond was not given intentional doping such as boron. The resistance at the contact between the diamond and an electrode, however, was very high. Because of this, it was very difficult to control the distance between electrodes so that a relatively high voltage is needed as a bias voltage to drive the prior art device and therefore the characteristics of devices were substantially dispersed.
By introducing boron ions into the diamond, good ohmic low resistant contacts can be obtained. The thermistor coefficient of the device, however, is decreased to be about 2000 (activation energy=0.21 eV) in case of 300 ppm doping of boron. Therefore, a need exists for diamond thermistors for forming good ohmic contacts without sacrifice of the thermistor coefficient.